As shown in FIG. 9(a) and (b) of Japanese Patent Application No. 3-239147, such an electromagnetic absorber in broad bands has been publicly known as the abovementioned electromagnetic absorber for broad bands, wherein sintered ferrite magnetic bodies attached on a metallic reflection plate are such that, in respective lattice bar portions each having an interval of an appointed width, a plurality of absorbing portions having a rectangular shape in its section, the lateral widths of which are different from each other, are laminated in multi-stages and integrally molded so that the lateral widths thereof become smaller from the base to the upper end.
That is, it is necessary that the thickness of a sintered ferrite magnetic body is established in compliance with a frequency of electromagnetic waves to be absorbed, in detail, .lambda./4, wherein, in order to enable electromagnetic absorption in broad bands, sintered ferrite magnetic bodies whose thickness is established for each of the frequency bands are combined to construct an electromagnetic wave absorber.
The sintered ferrite magnetic bodies are produced by baking a ferrite material, in which Ni-Zn ferrite powder or Mn-Zn ferrite powder are blended with a binder, after molding the same in the form of the multistage lattice in a press-molding process. However, since the sintered ferrite magnetic body is of a complicated shape having an undercut portion, it is difficult to draw the sintered ferrite magnetic body from a mold in the press-molding process, wherein the ratio of defectives was high.
And, in order to correspond to various types of frequency bands, it is necessary that the electromagnetic wave absorber has a plurality of absorbing portions, whose lateral widths are different from each other, laminated in multi-stages. Therefore, it is unavoidable that the weight of the electromagnetic wave absorber is increased, whereby construction efficiency thereof was remarkably low when attaching such electromagnetic wave absorbers to an electric wave darkroom or a building, etc.,
Further, where electromagnetic waves are brought into contact with the planes of the respective absorbing portions at a low incident angle, and since, in the electromagnetic wave absorbers for broad bands, the respective absorbing portions of the lattice bar portions are formed flat or plane, some part of the electromagnetic waves are reflected, wherein the ratio of electromagnetic permeation is low, and the electromagnetic wave absorption efficiency was unsatisfactory.
It is therefore an object of the invention to provide an electromagnetic wave absorber in broad bands, which can be efficiently produced by a simple method and resultantly can reduce production cost thereof.
It is another object of the invention to provide an electromagnetic wave absorber in broad bands having a reduced weight, for which working efficiency is excellent.
It is still another object of the invention to provide an electromagnetic wave absorber having an excellent ratio of electromagnetic wave absorption.